The present invention relates to a contention-based multiple access protocol for an uplink channel and, more particularly, to a contention-based multiple access protocol that allows variable-rate and multi-packet transmission on the uplink channel by user terminals.
In a wireless packet data network, a plurality of user terminals transmit packet data to an access point over a shared uplink channel. A random access protocol is often used to share a portion of the uplink bandwidth among the user terminals. The random access protocol may be a reservation-based protocol or a contention-based protocol. In either case, the channel is typically divided in the time domain into a sequence of time slots. The user terminals share the channel by transmitting in different time slots. The channel may also use Orthogonal Frequency Division Multiplexing (OFDM) or Code Division Multiple Access (CDMA) to allow multiple user terminals to transmit in the same time slot. In such cases, multiple user terminals may transmit in the same time slot, but on different subcarrier frequencies or with different spreading codes.
A reservation-based random access protocol reserves resources for individual user terminals. The mobile stations request permission from the access point to transmit on the shared uplink channel. If the request is granted, the access point reserves resources for the user terminal and sends a grant message to the user terminal identifying the reserved resources. The reserved resources may, for example, comprise a time slot or portion of a time slot. After receiving permission, the user terminal transmits its data using the allocated resources. One shortcoming of reservation-based multiple access protocols is the delay incurred in the request/grant procedure.
In contention-based multiple access protocols, resources are not reserved and the user terminals compete with one another for access to the channel. One such protocol is called slotted ALOHA. In slotted ALOHA, the shared uplink channel is divided into a sequence of time slots. When a user terminal has data to transmit, it selects a time slot and begins its transmission at the start of the selected time slot. With single packet reception, the packet will be received by the access point if no other user terminal transmits in the same slot. However, if another user terminal transmits in the same time slot, a collision occurs and neither packet will be received. In the event of a collision, each user terminal backs off a random amount and retransmits in another time slot. It has been shown that the maximum throughput using slotted ALOHA is 0.36 packets per slot. This low throughput is the main disadvantage of the slotted ALOHA approach.
Multi-packet reception (MPR) can be used to significantly improve the throughput of the slotted ALOHA approach. With MPR, the access point can receive multiple packets in the same time slot and frequency without collision. A number of techniques can be used to enable MPR including use of multiple receive antennas at the access point, code multiple access (CDMA) techniques, and multi-user detection techniques. The recently proposed Dynamic Queue Protocol exploits MPR capability of the receiver to provide an efficient access scheme.
MPR protocols developed to date have concentrated on the symmetric case where all packets have the same probability of reception. To achieve symmetry, it is assumed that power control is employed so that all packets are received with the same power by the access point. This approach, however, results in a very low system throughput because the user terminals in advantageous channel conditions are penalized.